1. Field of the Invention
The present invention relates to an apparatus for receiving GPS signals. Particularly, the present invention relates to an apparatus for receiving GPS signals, in which GPS signals are received by utilizing a non-coherent DLL method.
2. Description of the Prior Art
Generally, the GPS (global positioning system) is a system which consists of 24 satellites launched by the U.S. army, ground control stations and users' movable stations. The planar position of a movable station can be known by measuring the distances between the movable station and three or more of satellites.
If the position is to be measured by means of the GPS, the waves of four GPS satellites have to be simultaneously received, and the code pattern which is assigned to each of the satellites has to be decoded. The waves of a GPS satellite have a form of random secret codes, and therefore, when the waves are received, they are decoded by referring to a random code information. The waves which are transmitted by the GPS satellite correspond to a PRN code (pseudo random noise code), and they are signals which are spectrum-spread by a PSK (phase shift keying) modulation. The PRN code consists of two kinds, i.e., a C/A (coarse and access) code and a P (precision) code. The C/A code has been laid open, while the P code is handled secretly.
The signals which are transmitted by the GPS includes an L1 carrier wave of 1.5 GHz and an L2 carrier wave of 1.2 GHz. The signals which are received from the GPS satellite are down-converted two times, and among these signals, the C/A codes are inversely spread. In this way, the unique code of the relevant satellite is removed, thereby obtaining the desired information.
In the GPS satellite signals, the spectrum spreading method is used, and therefore, if the signals are not synchronized, communications are impossible. Therefore, in a non-synchronized state, the C/A code has to be synchronized within a short period of time. The active type synchronizing circuit which is capable of removing the C/A code within a short period of time includes typically three kinds.
A coherent delay lock loop demodulates basic signals after taking the correlation between the codes, in a state with the carrier waves removed from the received signals. U.S. Pat. No. 5,600,656 discloses an apparatus for receiving GPS signals by utilizing a coherent delay lock loop. In the conventional apparatus, the delay time of the C/A codes which is caused by the ionospheric layer is corrected by utilizing the delay time between the P codes of the L1 band and the L2 band.
A .tau.-dithering loop is a circuit having a single correlator, and thus, it is simple. In this circuit, the received signals are demodulated by time-dividing the leading signals and the lagging signals. The coherent delay lock loop is incapable of processing the intermediate frequency. In the .tau.-dithering loop, the circuit is simple, but the received signals are demodulated by time-dividing the leading signals and to lagging signals as described above, and therefore, it is weak against thermal noises. Further, a 6 dB correlation loss is generated compared with the DLL.
Therefore, there is a need for a GPS signal receiving apparatus in which a non-coherent delay lock loop is utilized so as to avoid the disadvantages of the .tau.-dithering loop and the coherent delay lock loop.